Eligibility for escape analysis / stack allocation with Java 7

Question

I am doing some tests with escape analysis in Java 7 in order to better understand what objects are eligible to stack allocation.

Here is the code I wrote to test st

Answer 1

I just investigated the same thing, but for Java 8. I put my answer in a duplicate question as I didn't find this one in time.

Summary from the full answer:

First of all, it's implementation dependent. This answer applies to OpenJDK 1.8 and probably also the Oracle JVM 1.8.

Secondly, as others have stated, stack allocation only happens when a method is compiled by the C2 compiler, which only happens once a method has been called enough times.

If so, objects can be stack allocated if

• all method calls that use it are inlined
• it is never assigned to any static or object fields, only to local variables (parameters to inlined method calls become local variables)
• at each point in the program, which local variables contain references to the object must be JIT-time determinable, and not depend on any unpredictable conditional control flow.
• If the object is an array, its size must be JIT-time constant and indexing into it must use JIT-time constants.

The inlining especially is not predictable if you don't know some of the specific quirks of Hotspot. See the linked answer for some details.

Edit: I tried running your test on java 8 (OpenJDK), and everything is inlined there. So there are differences in stack allocation between java 7 and 8.